Paraffin Isomerization units in refineries are used to convert low octane C5/C6 streams into high-octane products and to produce i-C4 for motor alkylation or methyl tert-butyl ether (MTBE) production. Chlorided alumina catalysts are commonly used in a light paraffin isomerization process such as the UOP Butamer and Penex processes. These catalysts are irreversibly deactivated by water or oxygenate precursors thereof (CO, CO2, methanol, etc.). Most paraffin isomerization units are equipped with dryers to remove moisture, to slow catalyst deactivation. In addition, traces (ppm concentrations) of sulfur compounds such as H2S, mercaptans, sulfides and thiophenes are poisons for isomerization catalysts. Therefore, guard beds based mainly on supported metal oxides are sometimes installed in front of the feed dryers. At the operating temperatures, typically 93° to 204° C. (200° to 400° F.) of the sulfur guard beds, the metal oxides reduce rapidly, thereby evolving large quantities of water which can overwhelm the feed driers and damage the expensive isomerization catalysts. The present invention involves an improvement to the feed purification of the paraffin isomerization units which use supported copper oxide (optionally, in addition to other metal oxides) for trace sulfur removal.
Guard beds with supported copper oxide (CuO) are often used for feed purification in the isomerization units. Unfortunately, the CuO reduces in the presence of the hot hydrocarbons, at the typical operating temperatures in the range of 93° to 204° C. (200° to 400° F.), which causes conversion of CuO to Cu2O and even to Cu metal, thereby producing water as reaction product. Typically the reduction of CuO occurs rapidly, and large amounts of water are produced. The excessive moisture could even overcome the down stream feed dryers and water leakage from the driers will cause irreversible catalyst deactivation.
Copper containing materials are widely used in industry as catalysts and sorbents. The water shift reaction in which carbon monoxide is reacted in presence of steam to make carbon dioxide and hydrogen as well as the synthesis of methanol and higher alcohols are among the most practiced catalytic processes nowadays. Both processes employ copper oxide based mixed oxide catalysts.
Copper-containing sorbents play a major role in the removal of contaminants, such as sulfur compounds and metal hydrides, from gas and liquid streams. One new use for such sorbents involve the on-board reforming of gasoline to produce hydrogen for polymer electrolyte fuel cells (PEFC). The hydrogen feed to a PEFC must be purified to less than 50 parts per billion parts volume of hydrogen sulfide due to the deleterious effects to the fuel cell of exposure to sulfur compounds.
Copper oxide (CuO) normally is subject to reduction reactions upon being heated but it also can be reduced even at ambient temperatures in ultraviolet light or in the presence of photochemically generated atomic hydrogen.
The use of CuO on a support that can be reduced at relatively low temperatures is considered to be an asset for some applications where it is important to preserve high dispersion of the copper metal. According to U.S. Pat. No. 4,863,894, highly dispersed copper metal particles are produced when co-precipitated copper-zinc-aluminum basic carbonates are reduced with molecular hydrogen without preliminary heating of the carbonates to temperatures above 200° C. to produce the mixed oxides. However, easily reducible CuO is disadvantageous in some important applications, such as the removal of hydrogen sulfide from gas and liquid streams when very low residual concentration of H2S in the product is required
The residual H2S concentration in the product gas is much higher (which is undesirable) when the CuO reduces to Cu metal in the course of the process since reaction (1) is less favored than the CuO sulfidation to CuS.2Cu+H2S=Cu2S+H2  (1)The known approaches to reduce the reducibility of the supported CuO materials are based on combinations with other metal oxides such as Cr2O3. The disadvantages of the approach of using several metal oxides are that it complicates the manufacturing of the sorbent because of the need of additional components, production steps and high temperature to prepare the mixed oxides phase. As a result, the surface area and dispersion of the active component strongly diminish, which leads to performance loss. Moreover, the admixed oxides are more expensive than the basic CuO component which leads to an increase in the sorbent's overall production cost.
The present invention comprises a new method to Improve feed purification in the light paraffin isomerization process by using supported CuO adsorbent which contains chloride as a means to decrease the tendency of CuO to be reduced to low valent state, especially Cu metal. Surprisingly, it has now been found that introducing chloride either in the basic copper carbonate, which serves as CuO precursor, or into the intermediate CuO-alumina adsorbent leads to material having improved resistance to reduction by hydrocarbons. This feature is useful in light paraffin isomerization process